UHMWPE, also known as high modulus PE or high performance PE, refers to linear polyethylene (PE) having a viscometrically measured average molecular weight of at least 1×106 g/mol. Due to its ultra high molecular weight (usually about 2 to 6 million atomic units), UHMWPE has excellent impact resistance, abrasion resistance, sliding properties, and chemical resistance. UHMWPE is widely employed in mechanical fields for example it is used in bearings, pulleys, and as a lining for trucks or dock guards. In the medical field, UHMWPE plays a prominent role as the only material that meets the requirements for tibia-inlays in modern knee implants. Use of this material, however, is more taxing than standard polymers. The ultra-high molecular weight of UHMWPE makes the product difficult to palletize. Thus, bulk density, particle size, and particle size distribution of UHMWPE are essential to the processability and use of this polymer which must be delivered in powder form, rather than the typical pellets of most polymers.
During UHMWPE manufacture, ethylene monomers are polymerized in the presence of a homogeneous (metallocene) or heterogeneous catalyst. Heterogeneous Ziegler-Natta (ZN) catalysts are commonly used in commercial scale production while homogeneous catalysts are used in rare cases where specific properties, such as a very narrow molecular weight distribution, are required. Heterogeneous ZN catalysts are comprised of the reaction product of a transition metal halide and an organometallic compound. Immobilization of the ZN catalyst on an oxide or halide compound increases catalytic activity and improves morphology.
Heterogeneous systems, however, have several drawbacks. Significantly, the surface of a heterogeneous catalyst particle may have multiple active sites each site having a different level of activity and selectivity towards monomer insertion. Multiple active sites on catalyst particles may result in uncontrolled morphology, reactor fouling, and inconsistent particle size (e.g., a large span value) and density. One potential cause of multiple active sites on a catalyst particle may be the way in which the transition metal halide binds to the support. Formation of the support dramatically affects the manner in which the transition metal halide molecule binds to form the final catalyst. Therefore control of support formation is essential to catalyst formation. Poor support formation leads to poor control over polymer structure.
There are various methods for preparing ZN catalysts in order to improve the properties and/or characteristics of these catalysts and subsequently improve the resulting polymer. More specifically, experiments and studies have been performed in relation to processes and methods for preparing specific catalysts with the aim of improving the properties and/or characteristics said catalysts.
United Stated Patent 2004/0063572 A1 discloses a method of forming a titanium solid catalyst impregnated on a particulate transitional metal compound and supported on inert solid particularly magnesium chloride. The method includes: (1) preparing magnesium compound solution; (2) reacting said compound solution with transitional metal compound or mixture thereof to produce a support; and (3) reacting said support with titanium compound and electron donor to produce solid complex titanium catalyst. However, the resulting catalyst has a large particle size and a fairly low bulk density, making this catalyst less beneficial for use with UHMWPE.
The method of U.S. patent application Ser. No. 13/041,028 uses a metallocene catalyst to form UHMWPE. U.S. patent application Ser. No. 12/971,843 forms a supported metallocene catalyst for preparation of HMW or UHMWPE. Metallocene catalysts have a single catalytic site and produce a polymer with a narrow molar mass composition and tacticity distribution. However, metallocene catalysts are very costly and the high aluminum to transition-metal ratio required to achieve catalytic activity make use of metallocenes prohibitive.
The catalyst of United States Patent 2006/0111523 is formed by an emulsion technique. This method forms the emulsion by dispersing the magnesium compound in a continuous phase of immiscible media. The resulting catalyst demonstrates controlled morphology. However, this method requires over 11 hours to complete and multiple additions and extractions making it costly and time consuming.
Various limitations, drawbacks, and/or disadvantages are associated with existing methods and processes for preparing catalysts for polymerizing olefins (i.e., preparing polyolefins). For example, lack of specific control of support formation in the foregoing methods may result in large or non-uniform catalyst particles. Non-uniform catalysts may result in polymers lacking uniformity in shape and size, having a broad size distribution. Formation of the support, the catalyst particle, and the UHMWPE may be time consuming, and/or costly. Therefore, there is a demand for more cost efficient and/or simpler methods and processes for preparing catalysts with improved morphological properties and/or enhanced catalytic activities.